Control apparatus and control method for multi-cylinder engine

ABSTRACT

In a multi-cylinder engine having a compression ignition combustion mode, a vibration detecting sensor that is preferably mounted in a cylinder block or a cylinder head is used to detect a frequency and the detected frequency is appropriately analyzed to detect or estimate a cylinder pressure peak value and peak timing for each cylinder. An amount of internal EGR, a fuel injection condition, an engine speed and the like are then controlled so as to bring each of these parameters into an appropriate range thereof. The control apparatus suppresses variations in combustion states among different cylinders and different cycles arising from unit-to-unit variations or deterioration in the engine or part-to-part variations or deterioration in a component thereof.

BACKGROUND OF THE INVENTION

The present invention relates to a control apparatus and a controlmethod for a multi-cylinder engine and, more particularly, to a controlapparatus and a control method for a multi-cylinder engine thatsuppresses, in a favorable manner, variations in an ignition combustionstate among different cylinders during an engine operation in acompression ignition combustion mode.

There is known a multi-cylinder engine that works on a combustionprinciple, in which a premixture is compressed and self-ignited. Thistype of engine is, what is called, a homogeneous charge compressionignition combustion multi-cylinder engine. It is desired that this typeof engine be controlled in a favorable manner so that it performs aself-ignition combustion that exhibits high efficiency over a wideoperating range. For example, Japanese Patent Laid-open No. 2000-320333discloses a technique, in which valve timing of intake and exhaustvalves are controlled so as to provide a valve shutoff period (minusoverlap) in the cylinder. This minus overlap period and fuel injectiontiming are controlled in accordance with changing operating conditions,thereby controlling self-ignition combustion of a mixture.

The technique disclosed in the aforementioned publication is intended tocontrol various components of the engine according to engine operatingconditions. If, for example, there are part-to-part variations amongthose engine components or if any deteriorates, however, a symptom couldoccur, in which an ignition timing and combustion state vary from onecylinder to another and from one cycle to another even under the sameoperating conditions. This symptom could cause pre-ignition or misfireto occur in one or more cylinders of the engine. There is as a result aproblem arising, in which fuel consumption and the state of exhaustemissions are seriously aggravated.

In a commonly known spark ignition multi-cylinder (gasoline) engine,control is provided with the ignition timing of an ignition plug firstestablished. In a diesel engine, too, combustion control is providedwith the ignition timing for fuel spray first established. Both areequipped with an ignition trigger and capable of an extremely powerfuland highly robust combustion control. There has therefore been littlenecessity to identify detailed combustion states for each cylinder andprovide combustion control based the combustion states.

With the homogeneous charge compression ignition combustionmulti-cylinder engine, however, it is assumed that a premixture of airand fuel self-ignites as a piston compresses it. For that reason, thistype of engine is not provided with any powerful ignition trigger as inthe spark ignition engine or the like described earlier and a combustionperiod thereof is shorter than that of the spark ignition type or thelike. This very fact enables operation at high efficiency with lowexhaust emissions. At the same time, even the slightest variations inthe cylinder structure greatly affects a combustion condition, whicheventually aggravates fuel consumption and the state of exhaustemissions. This gives a good reason to believe that there is a cylinderor two, in which combustion does not occur normally, even if each of allcylinders is controlled in exactly the same manner. It is ascertainedthrough experiments conducted by the applicant.

SUMMARY OF THE INVENTION

The present invention has been made in view of the aforementionedproblem. It is therefore an object of the present invention to provide acontrol apparatus and a control method for a multi-cylinder enginehaving a compression ignition combustion mode as an operating mode inwhich variations in the combustion state among different cylinders anddifferent cycles arising from unit-to-unit variations or deteriorationin the engine or part-to-part variations or deterioration in a componentthereof can be suppressed.

To achieve the foregoing object, a control apparatus for amulti-cylinder engine according to one aspect of the present inventionis basically applicable to a multi-cylinder engine that is capable ofperforming compression ignition combustion, or selecting either thecompression ignition combustion or spark ignition combustion. Thecontrol apparatus is provided with a cylinder-specific combustion stateestimating means that estimates a combustion state for each cylinder anda combustion state control means that controls the combustion state foreach cylinder. The combustion state control means controls, during anengine operation in a compression ignition combustion mode, thecombustion state for each cylinder based on the combustion stateestimated by the cylinder-specific combustion state estimating meansduring the compression ignition combustion mode.

With this arrangement, the control apparatus for a multi-cylinder engineaccording to the aspect of the present invention can estimate thecombustion state for each cylinder during the engine operation in thecompression ignition combustion mode. It is further able to provide acombustion control for each cylinder based on the estimated combustionstate for each cylinder. Should there be part-to-part variations amongcomponents of the multi-cylinder engine or should any deteriorate,therefore, the control apparatus can provide a best possible combustioncontrol for each cylinder in accordance with the part-to-partmanufacturing variations and deterioration to correct variations in thecombustion state among different cylinders and different cycles. Thiscan therefore realize compression ignition combustion offering good fuelconsumption and exhaust emissions characteristics.

A control apparatus for a multi-cylinder engine according to anotheraspect of the present invention is characterized in the followingpoints. That is, the cylinder-specific combustion state estimating meansestimates a peak value of a cylinder pressure of each cylinder and apeak timing of the cylinder pressure. The combustion state control meansprovides control so that the peak value of the cylinder pressure of eachcylinder falls within a predetermined range and/or the peak timing fallswithin a predetermined period of time after piston top dead center. Thecontrol apparatus is further characterized in that the combustion statecontrol means controls other combustion states, including a modechangeover between spark ignition combustion and compression ignitioncombustion and the like based on the operating condition of themulti-cylinder engine, the operating condition of a piece of equipmentmounted with the multi-cylinder engine, an intention of a user of theequipment and the like.

With this arrangement, the control apparatus for a multi-cylinder engineaccording to the aspect of the present invention can accurately estimatethe combustion state of each cylinder during an engine operation in thecompression ignition combustion mode. That is, if good combustion isbeing carried out in the compression ignition combustion mode, the peaktiming of the cylinder pressure coincides with peak timing of heatgeneration at predetermined timing after top dead center. It istherefore possible to estimate the combustion state of each cylinder bydetecting the cylinder pressure peak value and the peak timing. Althoughdepending on operating conditions, an operation at high efficiency canbe achieved if the peak timing falls within a predetermined range of 0and 10° after the top dead center.

A control apparatus for a multi-cylinder engine according to stillanother aspect of the present invention is characterized in thefollowing point. That is, the cylinder-specific combustion stateestimating means estimates the combustion state based on a signalprovided by at least either one of the following components: a vibrationdetecting sensor provided on a cylinder block or a cylinder head of themulti-cylinder engine; at least one rotary electric motor connecteddirectly or by way of a gear to an output shaft of the multi-cylinderengine; and, a cylinder pressure sensor disposed in at least one of theplurality of cylinders of the multi-cylinder engine.

As configured as described in the foregoing, the control apparatus for amulti-cylinder engine according to the aspect of the present inventionuses the vibration detecting sensor and, by appropriately analyzing afrequency of one vibration detecting sensor signal, can estimate thepeak value and the peak timing of the cylinder pressure individually foreach cylinder. As a result, the control apparatus can estimate thecombustion state for each cylinder during the engine operation in thecompression ignition combustion mode at low cost and appropriately. Byusing the rotary electric motor connected directly or by way of a gearto the output shaft of the multi-cylinder engine, the control apparatuscan detect highly accurately engine torque during the engine operationin the compression ignition combustion mode, which allows it to estimatethe combustion state of each cylinder. Furthermore, the controlapparatus may use the cylinder pressure sensor to take a direct readingof a highly accurate cylinder pressure for each cylinder, which makes itpossible to precisely estimate, from the cylinder pressure condition,the combustion state of each cylinder.

A control apparatus for a multi-cylinder engine according to a furtheraspect of the present invention is characterized in the following point.That is, the combustion state control means controls at least one of thefollowing mechanisms: a variable valve mechanism that varies the amountof internal EGR by changing at least one of the valve timing and valvelift amount of an intake valve and an exhaust valve; a fuel injectionmechanism that directly or indirectly injects fuel into a combustionchamber of the multi-cylinder engine; and, a transmission that transmitsoutput shaft torque of the multi-cylinder engine to a power trainmechanism of a vehicle through adjusting a gear ratio.

With the arrangement as described in the foregoing, the controlapparatus for a multi-cylinder engine according to the aspect of thepresent invention can control the combustion state so that the peakvalue and the peak timing of the cylinder pressure of each cylinder fallwithin an appropriate range during the engine operation in thecompression ignition combustion mode. The control apparatus accomplishesthis task by controlling the variable valve mechanism, the fuelinjection mechanism, and the transmission, either independently orcoordinately, according to a command issued by the combustion statecontrol means, should necessity arise to change the combustion state ofeach cylinder as determined based on the results of estimation made bythe cylinder-specific combustion state estimating means.

A control apparatus for a multi-cylinder engine according to a stillfurther aspect of the present invention is characterized in thefollowing points. In one, the combustion state control means controlsthe variable valve mechanism so as to increase a minus overlap amount,thereby increasing the amount of internal EGR and/or the transmission soas to change the gear ratio, thereby decreasing an engine speed. Thecontrol apparatus performs the aforementioned control operations when itis estimated that the peak value of cylinder pressure of at least one ofthe plurality of cylinders is lower than the predetermined value, or thepeak timing of cylinder pressure is retarded in relation to thepredetermined timing, based on the estimation made by the cylindercombustion state estimating means. In the other, the combustion statecontrol means controls the variable valve mechanism so as to decreasethe minus overlap amount, thereby decreasing the amount of internal EGRand/or the transmission so as to change the gear ratio, therebyincreasing the engine speed. The control apparatus performs theaforementioned control operations when it is estimated that the peakvalue of cylinder pressure of at least one of the plurality of cylindersis higher than the predetermined value, or the peak timing of cylinderpressure is advanced in relation to the predetermined timing, based onthe estimation made by the cylinder combustion state estimating means.

With the arrangement as described in the foregoing, the controlapparatus for a multi-cylinder engine according to the aspect of thepresent invention can appropriately control the peak value and peaktiming of the cylinder pressure for each cylinder. It can therebysuppress variations in the combustion state among different cylindersand different cycles during the engine operation in the compressionignition combustion mode, thus realizing compression ignition combustionoffering good fuel consumption and exhaust emissions characteristics.

Specifically, it is determined whether the peak timing of each cylinderis advanced or retarded in relation to the timing within thepredetermined range. If it is determined that the peak timing isadvanced, the variable valve is controlled so as to increase the amountof internal EGR, thus improving ignitability of mixtures in allcylinders, that is, advancing the ignition timing. If the engine speedis relatively high (2,000 revolutions per minute or more), however,compression ignition combustion can at times fail due to the amount ofintake air being decreased as the internal EGR amount increases. Thetransmission is therefore controlled so as to slightly decrease theengine speed, thereby increasing the amount of fuel per one cycle for agreater ignitability. If it is determined that the peak timing isretarded, the variable valve is controlled so as to decrease the amountof internal EGR, thereby retarding the ignition timing for allcylinders. At this time, the transmission may be controlled so as toslightly increase the engine speed, thereby decreasing the amount offuel per one cycle to provide an equivalent power output.

It is next determined whether or not the peak value of each cylinderfalls within the predetermined range. If it is determined that the peakvalue is lower, the variable valve is controlled so as to increase theamount of internal EGR, thereby improving ignitability of the mixturesof all cylinders, thus allowing a condition, in which heat tends to begenerated from combustion, to develop. As noted earlier, if the enginespeed is relatively high (2,000 revolutions per minute or more),compression ignition combustion can at times fail due to the amount ofintake air being decreased as the amount of internal EGR increases. Atthis time, therefore, the transmission may also be controlled so as toslightly decrease the engine speed, thereby increasing the amount offuel per one cycle to provide an equivalent power output.

A control apparatus for a multi-cylinder engine according to a yetfurther aspect of the present invention is characterized in thefollowing points. Specifically, the combustion state control meanseither increases or decreases the amount of fuel supplied from the fuelinjection mechanism to each cylinder per one cycle. The fuel injectionmechanism is a type that injects fuel directly into the cylinder. Thecombustion state control means provides control so as to effect at leastone fuel injection during a minus overlap period by the variable valvemechanism during the compression ignition combustion mode. If thecylinder-specific combustion state estimating means estimates that thecylinder pressure peak value of at least one cylinder out of theplurality of cylinders is lower than the predetermined value, or thecylinder pressure peak timing of at least one of the plurality ofcylinders is retarded in relation to the predetermined timing, thecombustion state control means controls the fuel injection mechanism toincrease the amount of fuel injected from the mechanism into thecylinder during the minus overlap period. Alternatively, if the cylindercombustion state estimating means estimates that the cylinder pressurepeak value of at least one cylinder out of the plurality of cylinders ishigher than the predetermined value, or the cylinder pressure peaktiming of at least one of the plurality of cylinders is advanced inrelation to the predetermined timing, the combustion state control meanscontrols the fuel injection mechanism to decrease the amount of fuelinjected from the mechanism into the cylinder during the minus overlapperiod.

With the arrangement as described in the foregoing, the controlapparatus for a multi-cylinder engine according to the aspect of thepresent invention can arbitrarily control the condition of the mixtureby varying the injection timing or the like according to operatingconditions, if the direct fuel injection system is employed. The controlapparatus may be applied to a system in which a fuel injection sequenceis carried out a plurality of times during the minus overlap period soas to make part of the fuel radical, and the ratio of injection isvaried among different cylinders, which allows the apparatus to suppressvariations in the combustion state among different cylinders.

A control apparatus for a multi-cylinder engine according to a yetfurther aspect of the present invention is characterized in thefollowing points. Specifically, if the cylinder-specific combustionstate estimating means estimates that the cylinder pressure peak valueof at least one cylinder out of the plurality of cylinders falls outsidethe predetermined range, or the cylinder pressure peak timing of atleast one of the plurality of cylinders falls outside the predeterminedperiod of time, the control apparatus inhibits the compression ignitioncombustion mode for the multi-cylinder engine. Alternatively, thecontrol apparatus switches the operating mode to the spark ignitioncombustion mode. The control apparatus according to this aspect is alsocharacterized in that it warns the user of the equipment mounted withthe multi-cylinder engine, if the control apparatus inhibits thecompression ignition combustion mode.

The cylinder pressure peak value of the cylinder may still fall outsidethe predetermined range, or the cylinder pressure peak timing may remainoutside the predetermined period of time even through combustion statecontrol. In such cases, it could become impossible to run the engine inthe compression ignition combustion mode with high efficiency and lowexhaust emissions because of a deteriorated or defective engine part. Ifthis happens, the control apparatus for a multi-cylinder engineaccording to the aspect of the present invention can, thanks to thearrangement as described in the foregoing, inhibit the engine operationin the compression ignition combustion mode and suspend the operation.Alternatively, the control apparatus can switch to the spark ignitioncombustion mode and warns the user that the engine is no longer able tocarry out high efficiency operation.

A control method for a multi-cylinder engine according to one aspect ofthe present invention is substantially one for performing compressionignition combustion control, or control to select either the compressionignition combustion or spark ignition combustion. During an engineoperation in the compression ignition combustion mode, the methodestimates a combustion state for each cylinder and controls thecombustion state for each cylinder based on the estimated combustionstate for each cylinder. The control method is further characterized bycontrolling other combustion states, including a mode changeover betweenspark ignition combustion and compression ignition combustion, and thelike based on the operating condition of the multi-cylinder engine, theoperating condition of a piece of equipment mounted with themulti-cylinder engine, an intention of a user of the equipment and thelike.

In a control method for a multi-cylinder engine according to anotheraspect of the present invention, the step of estimating the combustionstate of each cylinder is to estimate a peak value of a cylinderpressure for each cylinder and a peak timing of the cylinder pressure.The step of controlling the combustion state is to ensure that the peakvalue of the cylinder pressure for each cylinder falls within apredetermined range and/or the peak timing falls within a predeterminedperiod of time after piston top dead center.

In a control method for a multi-cylinder engine according to stillanother aspect of the present invention, the cylinder pressure of eachcylinder represents a value that is the result of analyzing a vibrationfrequency detected of the multi-cylinder engine, or a value of acylinder pressure sensor disposed in at least one of the plurality ofcylinders of the multi-cylinder engine.

A control method for a multi-cylinder engine according to a furtheraspect of the present invention controls compression ignition combustionor controls to select either the compression ignition combustion andspark ignition combustion. During the engine operation in thecompression ignition combustion mode, the method estimates thecombustion state for each cylinder during the engine operation in thecompression ignition combustion mode. If at least either one of thefollowing conditions is estimated, the method allows a variable valvemechanism of the cylinder to increase or decrease a minus overlap amountso as to increase or decrease an internal EGR amount, therebycontrolling the combustion state. The conditions are that the peak valueof the cylinder pressure of at least one cylinder of all the pluralityof cylinders is lower or higher than a predetermined value, and that thepeak timing of the cylinder pressure is retarded or advanced in relationto a predetermined timing. Furthermore, if at least either one of thefollowing conditions is estimated, the method allows a transmission tochange a gear ratio so as to increase or decrease an engine speed,thereby increasing or decreasing the amount of fuel supplied from a fuelinjection mechanism to each cylinder per one cycle. The conditions arethat the peak value of the cylinder pressure of at least one cylinder ofall the plurality of cylinders is lower or higher than the predeterminedvalue, and that the peak timing is retarded or advanced in relation tothe predetermined timing.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent fromthe following description of embodiments with reference to theaccompanying drawings in which:

FIG. 1 is a system configuration diagram showing a multi-cylinder enginecontrol apparatus according to a first embodiment of the presentinvention;

FIGS. 2A to 2D are concerned with the multi-cylinder engine controlapparatus shown in FIG. 1, FIG. 2A showing schematically a vibrationdetecting sensor mounted on the multi-cylinder engine, FIG. 2B showingoutput characteristics of the vibration detecting sensor, FIG. 2Cshowing a cylinder pressure history during the multi-cylinder engineoperation in the compression ignition combustion mode, and FIG. 2Dshowing a history of output values detected by the vibration detectingsensor during the multi-cylinder engine operation in the compressionignition combustion mode;

FIG. 3 shows a flowchart for control operations performed by themulti-cylinder engine control apparatus while the engine is running inthe compression ignition combustion mode;

FIG. 4 shows a flowchart for control operations performed by themulti-cylinder engine control apparatus for variable valves and enginespeed based on the cylinder pressure peak timing and peak value;

FIG. 5 shows typical lift curve characteristics of the intake andexhaust variable valves used in the multi-cylinder engine controlapparatus shown in FIG. 1;

FIGS. 6A and 6B show a typical fuel injection control technique employedin the multi-cylinder engine control apparatus shown in FIG. 1, FIG. 6Ashowing the timings at which the intake and exhaust variable valves areopened and closed and fuel is injected (a plurality of times) and FIG.6B showing the relationship between the first fuel injection amount andthe compression ignition timing;

FIGS. 7A and 7B show the cylinder pressure peak timing and peak value ofeach cylinder while the multi-cylinder (four-cylinder) engine is runningin the compression ignition combustion mode, FIG. 7A(i) showing wherethe cylinder pressure peak timing of a specific cylinder is retarded,FIG. 7A(ii) showing where the state of FIG. 7A(i) has been correctedthrough combustion control, FIG. 7B(i) showing where the cylinderpressure peak value of a specific cylinder is low, and FIG. 7B(ii)showing where the state of FIG. 7B(i) has been corrected throughcombustion control;

FIG. 8 is a system configuration diagram showing a multi-cylinder enginecontrol apparatus according to a second embodiment of the presentinvention;

FIG. 9 is a system configuration diagram showing a multi-cylinder enginecontrol apparatus according to a third embodiment of the presentinvention; and

FIG. 10 is a system configuration diagram showing a multi-cylinderengine control apparatus according to a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A control apparatus and a control method for an engine according topreferred embodiments of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a system configuration diagram showing an engine controlapparatus according to a first embodiment of the present invention. Anengine 12 is a multi-cylinder engine. The multi-cylinder engine 12 isconfigured so as to carry out combustion in either of two differentmodes: a spark ignition combustion using an ignition system and acompression ignition combustion, in which a mixture is self-ignited bypiston compression.

Specifically, the multi-cylinder engine 12 is capable of performing ahigh-efficiency operation using the compression ignition combustion modein a low-speed, light-load range. More specifically, the range is anengine operating range of up to about a half of full load in terms of anengine torque and about 3,000 revolutions per minute, or rpm, in termsof an engine speed. It performs spark ignition combustion in any otherload ranges.

The engine 12 is provided with a vibration detecting sensor 5 disposedin a cylinder block or a cylinder head that forms a plurality ofcylinders thereof. In addition, there are disposed an air flow sensor 6on an upstream side where air is drawn in, a throttle 7 locateddownstream therefrom for adjusting the amount of air, an intake port 8,a fuel injection valve (hereinafter referred to as an “injector”) 9provided for each of the plurality of cylinders, and a speed sensor 13.Preferably, the injector 9 is a type that directly injects fuel into acombustion chamber 11 of each cylinder and the throttle 7 is anelectronic control throttle, in which a throttle valve is driven by anelectric actuator.

The engine 12 is further provided with variable valves 16 a, 16 b and apower train control unit (control device) (hereinafter referred to asthe PCU) 1.

Driving a starter 10 starts the operation of the engine 12. An outputshaft 15 of the engine 12 is connected to a transmission 17 of a vehiclethrough a friction clutch 14. It is further connected by way of atransmission output shaft 18, a reduction gear 19 and the like to awheel 20. It is desirable that the transmission 17 be a continuouslyvariable transmission type (hereinafter referred to as a CVT), withwhich a required speed can easily be achieved.

The PCU (control device) 1 receives inputs of a vehicle speed Vs, abrake signal Sb, an accelerator opening α, a gear ratio (gear position)Mp, a transmission output shaft speed Nout and the like as means ofrecognizing (detecting) a vehicle operating condition and a user'sintention. The PCU 1 also receives, as means of identifying an engineoperating condition, inputs of an air-fuel ratio A/F, an engine intakeair temperature Tin, an engine exhaust gas temperature Tex, anengine-cooling water temperature Tw, an engine vibration Pe suppliedfrom the vibration detecting sensor 5, an engine speed Ne, a throttleopening θtp and the like.

The PCU (control device) 1 functions to determine the controlled statesof the engine 12, the transmission 17 and the like. The PCU 1 isprovided at least with a combustion state control means including acylinder-specific combustion state estimating means 2 for thecompression ignition combustion mode, a fuel injectioncondition/variable valve condition setting means 3, and a gear ratiosetting means 4.

The cylinder-specific combustion state estimating means 2 uses datarepresenting the speed Ne of the engine 12, the vibration Pe provided bythe vibration detecting sensor 5 and the like to estimate a combustionstate of each cylinder. The cylinder combustion state estimating means 2then outputs data indicating the estimated combustion state to the fuelinjection condition/variable valve condition setting means 3 and thegear ratio setting means 4. The PCU 1 makes the means 2 to 4 performvarious calculations based on values detected by the sensors and thelike. Based on the results of these calculations, the PCU estimates thecontrolled state of each cylinder to realize a favorable compressionignition combustion mode.

The injector 9 injects a predetermined amount of fuel according to atarget engine torque that is calculated using the opening signal θtp ofthe throttle 7, the accelerator opening signal α and the like. Thevariable valve 16 a and the variable valve 16 b vary valve timing and anamount of valve lift on an intake side and an exhaust side,respectively. They are optimally controlled either hydraulically orthrough a motor current according to the operating condition. An actualoperation control system will be described later.

FIG. 2A through FIG. 2D show the vibration detecting sensor 5 used asthe combustion state estimating means for each cylinder, characteristicsthereof, and an example of detection made by the sensor.

FIG. 2A shows schematically the vibration detecting sensor 5 mounted onthe engine 12. The vibration detecting sensor 5 is composed of atransducer 5 a as a weight and a piezoelectric device 5 b. The sensor 5is secured to a cylinder block or a cylinder head 12 a of the engine 12.The piezoelectric device 5 b detects a voltage that corresponds tovibration of the transducer 5 a that represents the vibration frequencyof the engine 12.

FIG. 2B shows output characteristics of the vibration detecting sensor 5representing the relation between vibration frequencies of the engine 12and output voltages of the sensor 5. FIG. 2C shows the relation betweencylinder pressures during operations of the engine 12 in the compressionignition combustion mode and crank angles. FIG. 2D shows the relationbetween output values of the vibration detecting sensor 5 duringoperations of the engine 12 in the compression ignition combustion modeand crank angles.

The vibration detecting sensor 5 is ordinarily used as a detonationsensor in the spark ignition combustion mode. According to the firstembodiment of the present invention, however, the vibration detectingaccuracy of the piezoelectric device 5 b that contributes to the outputof the sensor is more enhanced as compared with that for detonationdetection. The frequency of the detected vibration is thus analyzed witha greater accuracy. This allows cylinder pressure peak timing and a peakvalue in the compression ignition combustion mode to be estimated.

Specifically, if good combustion is carried out during the engineoperation in the compression ignition combustion mode, the cylinderpressure peak timing coincides with the peak timing of heat generationat predetermined timing after top dead center as shown in FIG. 2C.Detecting an engine vibration at the cylinder pressure peak timing will,therefore, allow the cylinder pressure peak value to be estimated usingan output voltage value corresponding to the engine vibration.

When the detected frequency is subjected to a Fourier analysis andsuperposed with time-of-day data, the peak timing and peak value foreach cylinder can be estimated. Estimation with even greater accuracy ispossible of the cylinder pressure peak timing and the peak value, if thedetected frequency is subjected to a wavelet transform. FIGS. 2C and 2Dshow an example of applying this approach. FIG. 2C shows a pressurewaveform in compression ignition combustion as detected by the cylinderpressure sensor. FIG. 2D shows an output waveform that is the result ofan output detected by the vibration detecting sensor 5 and calculation.As is known from the foregoing, the cylinder pressure peak timing andthe peak value can be estimated even with the vibration detecting sensor5.

Disposing the vibration detecting sensor 5 according to the embodimentat the cylinder block or the cylinder head 12 a of the engine 12 makesit possible to estimate the combustion state in the compression ignitioncombustion mode for each cylinder at low cost. It goes without sayingthat the vibration detecting sensor 5 according to the embodiment can beused as a detonation sensor in spark ignition combustion.

Control operations of the control apparatus for the engine according tothe first embodiment of the present invention will be described indetail.

FIG. 3 shows a flowchart for control operations performed while theengine 12 is running in the compression ignition combustion mode ascontrolled by the PCU (control device) 1.

In step 1001 s, the PCU 1 reads the vehicle operating conditions,signals indicating the user's intention including the acceleratoropening α of the engine 12 and the brake signal Sb, and the engineoperating conditions as indicated by the engine speed Ne and the like.Then in step 1002 s, the PCU 1 determines whether or not the engine 12is in an operating range for the compression ignition combustion modebased on the readings taken in step 1100 s.

When the PCU 1 determines that the engine 12 is in the operating rangefor the compression ignition combustion mode in step 1002 s, it thendetermines in step 1003 s whether or not the operation in thecompression ignition combustion mode is being carried out. If the PCU 1determines that the current operation is being carried out in the sparkignition combustion mode (that is, it is not the compression ignitioncombustion mode), it proceeds to step 1003 as and selects thecompression ignition combustion mode before proceeding to step 1004 s.If it determines that the current operation is being carried out in thecompression ignition combustion mode, it then directly proceeds to step1004 s.

In step 1004 s, the PCU 1 reads the output values from the sensors usedfor estimating the combustion state of each cylinder in the compressionignition combustion mode. In step 1005 s, the PCU 1 estimates thecylinder pressure peak timing and the peak value of each cylinder.Specific techniques used for estimating the cylinder pressure in steps1004 s and 1005 s will be detailed later.

Then, in step 1006 s, the PCU 1 uses the estimated values in steps 1004s and 1005 s to determine whether or not the cylinder pressure peaktiming of all cylinders falls within the predetermined range. Thepredetermined range for the cylinder pressure peak timing refers to anapproximate range from a point after the top dead center on thecompression stroke up to 10° thereafter. The range varies slightlydepending on the operating condition. It can nonetheless be determinedthat the combustion is carried out at proper timing as long as thecylinder pressure peak timing falls within this range in the compressionignition combustion mode.

If it is found that there is a cylinder, the pressure peak timing ofwhich does not fall within the predetermined range, the PCU 1 thenproceeds to step 1008 s and provides control for the combustion state(the control technique employed will be detailed later). In step 1009 s,the PCU 1 determines the amount of deviation in peak timing amongdifferent cylinders. If the amount of deviation is a predetermined valueor more, or more specifically, if the cylinder pressure peak timing ofall cylinders does not fall within the range from a point after the topdead center on the compression stroke up to 10° thereafter, thefollowing could result. That is, it could become impossible to carry outan operation with high efficiency and low exhaust emissions throughcompression ignition combustion due to a deteriorated or defectiveengine component. The PCU 1 therefore proceeds to step 1012 s andinhibits the operation in the compression ignition combustion mode. Inthe meantime, it proceeds to step 1013 s and notifies the user that theengine is unable to perform a high-efficiency operation. This sequenceof control operations is repeated several times to set the operationinto the compression ignition combustion mode after the engine has beenrestarted. If the PCU 1 determines that the compression ignitioncombustion mode should be inhibited in all of these sequences, it warnsthe user to have the equipment and engine serviced.

Going back to step 1006 s, if the cylinder pressure peak timing of allcylinders falls within the predetermined range, the PCU 1 proceeds tostep 1007 s and determines whether or not the cylinder pressure peakvalues of all cylinders fall within the predetermined range. If thecylinder pressure peak values of all cylinders fall within thepredetermined range, then the PCU 1 judges that compression ignitioncombustion is being carried out in a favorable manner.

If it is found that the cylinder pressure peak value of any of thecylinders falls outside the predetermined range, it can be judged thatthe combustion is not properly carried out. The PCU 1 then proceeds tostep 1010 s and provides control for the combustion state (the controltechnique employed will be detailed later). Proceeding then to step 1011s, the PCU 1 determines once again whether or not the cylinder pressurepeak values of all cylinders fall within the predetermined range. If itis found that any falls outside the range, control proceeds to steps1012 s and 1013 s as described earlier.

To sum up, the procedures performed to estimate and control thecombustion state for each cylinder effectively prevent fuel economy andexhaust emissions of the engine 12 and operating performance of theequipment mounted with the engine 12 from being aggravated seriouslyeven when the equipment (vehicle or the like) and the engine 12deteriorate or there are part-to-part variations among differentcomponents. It is therefore possible to provide a multi-cylinder engine12 that offers outstanding durability and robustness and a piece ofequipment mounted with the engine.

Techniques used to estimate and control the combustion state of eachcylinder of the engine 12 according to the first embodiment of thepresent invention will be detailed below with reference to FIGS. 4 to 6.

FIG. 4 shows a flowchart for control operations performed for thecombustion state control means in the compression ignition combustionmode for each cylinder of the engine 12 according to the firstembodiment of the present invention. The combustion state of eachcylinder is detected through the procedures described earlier and, basedon the detected conditions, compression ignition combustion iscontrolled for each cylinder.

Based on the estimated results of the combustion state of each cylinder,the PCU 1 determines in step 1014 s whether or not the cylinder pressurepeak timing of all cylinders falls within the predetermined range. If itis found that the timing falls within the predetermined range, it can beestimated that combustion is carried out at a proper timing in each ofall cylinders. Then, the PCU 1 proceeds to step 1019 s and determineswhether or not the cylinder pressure peak values of all cylinders fallwithin the predetermined range.

If it is found that the peak timing of any of the cylinders fallsoutside the predetermined range in step 1014 s, the PCU 1 proceeds tostep 1015 s and determines whether or not the peak timing of thespecific cylinder in question is advanced or retarded in relation totiming in the predetermined range. If it is found that the peak timingis retarded, the PCU 1 proceeds to step 1016 as and uses the variablevalves 16 a and 16 b to increase the amount of internal EGR, therebyenhancing ignitability of the mixture in all cylinders, that is,advancing the ignition timing. If the engine speed is relatively high(2,000 rpm or more), however, compression ignition combustion can faildue to a decreased amount of intake air as a result of the increasedamount of internal EGR. The PCU 1 may proceed to step 1016 bs in whichthe PCU 1 allows the gear ratio setting logic 4 to send a signal to thetransmission 17 to slightly decrease the engine speed, therebyincreasing the amount of fuel per one cycle for better ignitability,thus achieving an equivalent output. Although not shown in FIG. 4according to the embodiment, a supercharger device may be provided onthe intake side, in which case the amount of intake air may be increasedto boost the cylinder pressure, thereby advancing the ignition timing.

If it is found that the peak timing is advanced in step 1015 s, the PCU1 proceeds to step 1016 cs and uses the variable valves 16 a and 16 b todecrease the amount of internal EGR, thereby retarding the ignitiontiming for all cylinders. At this time, the PCU 1 may proceed to step1016 ds and allow the gear ratio setting logic 4 to send a signal to thetransmission 17 to slightly increase the engine speed, therebydecreasing the amount of fuel per one cycle, thus achieving anequivalent output.

After having brought the combustion state of each cylinder into or nearthe predetermined range through the steps from 1016 as to 1016 ds, thePCU 1 proceeds to step 1017 s. The PCU 1 therein provides an injectionratio control for each cylinder, thus narrowing the cylinder pressurepeak timings of different cylinders down to an even shorter period ofpredetermined timing to enable operations at even higher efficiency.

Proceeding to step 1018 s, the PCU 1 determines therein if the amount ofdeviation in the cylinder pressure peak timing among different cylindersis a predetermined value or more. If the amount of deviation is large, acylinder, in which combustion is retarded, tends to exhaust unburned HC,while a cylinder, in which combustion is advanced, tends to produce ahigh-pitch combustion noise or the engine can become easy to be damaged.If the amount of deviation is large, the PCU 1 proceeds to step 1024 sand inhibits the compression ignition combustion mode. At the same time,the PCU 1 notifies the user of the equipment that the engine is unableto perform a high-efficiency operation (in step 1025 s). The PCU 1allows the engine to enter the compression ignition combustion mode ifrequired conditions are met when the engine is restarted after thenotification. Determining that the compression ignition combustion modeshould be inhibited for a number of consecutive times, the PCU 1 warnsthe user to have the equipment and engine serviced.

If the PCU 1 determines in step 1018 s that the amount of deviationfalls within the predetermined range, the PCU 1 proceeds to step 1019 sto determine whether or not the cylinder pressure peak values of allcylinders fall within the predetermined range. If the cylinder pressurepeak values of all cylinders fall within the predetermined range, thenthe PCU 1 can estimate that combustion is being carried out at propertiming in all cylinders.

If it is determined in step 1019 s that the cylinder pressure peak valueof any of the cylinders falls outside the predetermined range, the PCU 1proceeds to step 1020 s and determines whether the peak value of thecylinder in question is higher or lower than the predetermined range. Ifthe peak value is lower, the PCU 1 proceeds to step 1021 as and uses thevariable valves 16 a and 16 b to increase the amount of internal EGR.This enhances ignitability of the mixtures in all cylinders, thusyielding a state in which heat generation tends to occur throughcombustion. If the engine speed is relatively high (2,000 rpm or more),however, compression ignition combustion can fail due to a decreasedamount of intake air as a result of the increased amount of internalEGR. Then, the PCU 1 may proceed to step 1021 bs, in which the PCU 1allows the gear ratio setting logic 4 to send a signal to thetransmission 17 to slightly decrease the engine speed, therebyincreasing the amount of fuel per one cycle for better ignitability,thus achieving an equivalent output. Although not shown in FIG. 4according to the embodiment, a supercharger device may be provided onthe intake side, in which case the amount of intake air may be increasedto boost the cylinder pressure, thereby enhancing ignitability.

If the peak value is found to be higher in step 1020 s, the PCU 1proceeds to step 1021 cs and uses the variable valves 16 a and 16 b todecrease the internal EGR amount, thereby decreasing the cylindertemperature so as to make heat generation through combustion milder. Atthis time, the PCU 1 may proceed to step 1021 ds and allow the gearratio setting logic 4 to send a signal to the transmission 17 toslightly increase the engine speed, thereby decreasing the amount offuel per one cycle, thus achieving an equivalent output.

After having brought the combustion state of each cylinder into or nearthe predetermined controlled range through steps 1021 as to 1021 ds, thePCU 1 proceeds to step 1022 s. The PCU 1 therein provides an injectionratio control for each cylinder, thus narrowing the cylinder pressurepeak timing of different cylinders down to an even shorter period ofpredetermined timing and bringing the peak values into the predeterminedrange to enable operations at even higher efficiency.

Proceeding to step 1023 s, the PCU 1 determines therein if the amount ofdeviation in the cylinder pressure peak value among different cylindersfalls within a predetermined range. If the amount of deviation fallsoutside the range, the PCU 1 proceeds to step 1024 s and inhibits thecompression ignition combustion mode. At the same time, the PCU 1notifies the user of the equipment that the engine is unable to performa high-efficiency operation (in step 1025 s). The PCU 1 allows theengine to enter the compression ignition combustion mode if requiredconditions are met when the engine is restarted after the notification.If the PCU 1 determines that the compression ignition combustion modeshould be inhibited for a number of consecutive times, the PCU 1 warnsthe user to have the equipment and engine serviced.

The combustion state control procedures as described in the foregoingfunction to smooth out varying combustion states among differentcylinders. This therefore prevents the combustion state from beingvaried due to variations and deterioration in engine components, thusenabling an operation with high efficiency and low exhaust emissions.

FIG. 5 shows typical lift curve characteristics of the intake andexhaust variable valves 16 a, 16 b used in the embodiment of the presentinvention. According to the embodiment, variable valves are used forintake and exhaust valves 16 a, 16 b to accomplish the following task.That is, the valve lift amount and the operating angle of these valvesare controlled as shown in FIG. 5 to vary a period of time during whichthe exhaust is shut off (hereinafter referred to as the minus overlapperiod), thereby continuously controlling the amount of internal EGR.

A solenoid driven valve may, in this case, be provided for each cylinderto continuously vary the minus overlap period. This, however, couldsubstantially drive up cost as the number of cylinders increases in themulti-cylinder engine. A configuration as detailed in the following istherefore provided to achieve the objects of the present invention. Thatis, there is provided a device that varies the valve lift amount and theoperating angle for all cylinders at the same time, with which the minusoverlap amount can be continuously variable depending on the operatingconditions.

FIGS. 6A and 6B show a typical fuel injection control technique employedaccording to the embodiment of the present invention. Since the fuelinjection system employed in the embodiment is direct injection, thecondition of the mixture can be controlled by changing the injectiontiming and the like according to the operating condition.

Referring to FIG. 6A, a fuel injection sequence is carried out aplurality of times. The first sequence is carried out during the minusoverlap period so as to make part of the fuel radical. The ratio ofinjection is then varied among different cylinders, thereby suppressingvariations in the combustion state among different cylinders.

Referring to FIG. 6B, in a range where the ratio of the amount of fuelinjected in the first injection sequence is large, combustion may occurduring the exhaust stroke or the intake stroke. Specifically, theinjection ratio control is therefore provided to ensure that the ratioof the amount of fuel injected in the first injection sequence does notexceed ¾ of the total amount of fuel injected.

FIGS. 7A and 7B show combustion states of a four-cylinder engine,representing the techniques used to estimate and control the combustionstate of each cylinder described in the foregoing as they are applied tothe operation of the four-cylinder engine in the compression ignitioncombustion mode.

FIG. 7A(i) shows a state in which the cylinder pressure peak timing ofcylinder no. 1 (#1) deviates from the predetermined range (that is,retarded) under a certain operating condition. When the combustion statecontrol technique described in the foregoing is applied thereto, thecylinder pressure peak timing of all cylinders falls within thepredetermined range, enabling an operation at high efficiency with lowexhaust emissions, as shown in FIG. 7A(ii). If the execution of thecombustion state control does not bring the cylinder pressure peaktiming of all cylinders into the predetermined range, it is difficult tokeep the engine operation at high efficiency with low exhaust emissions.The compression ignition combustion mode is then inhibited and the userof the equipment mounted with the engine is notified of it.

FIG. 7B(ii) shows a state in which the cylinder pressure peak value ofcylinder no. 1 (#1) is lower than the predetermined value under acertain operating condition. In such a case, the peak value and the peaktiming are not sufficiently clear and identifiable as depicted in FIG.7B(i). The engine is then likely to exhaust a large amount of unburnedHC and the like, with reduced combustion efficiency due to misfire andincomplete combustion. When the combustion state control technique asdescribed in the foregoing is applied thereto, the cylinder pressurepeak timing of all cylinders is brought into the predetermined range andthe peak values become the predetermined value or more as shown in FIG.7B(ii). It tells that the engine is running at high efficiency with lowexhaust emissions.

In the same manner as with FIG. 7A, if the execution of the combustionstate control does not bring the cylinder pressure peak timing of allcylinders into the predetermined range, or if the cylinder pressure peakvalue remains lower than the predetermined value even after theexecution of the control, it is difficult to keep the engine operationat high efficiency with low exhaust emissions. The compression ignitioncombustion mode is then inhibited and the user of the equipment mountedwith the engine is notified of it.

The engine 12 according to the embodiment of the present inventionemploys a direct injection system for fuel injection. A system, in whichfuel is injected into the intake pipe, may of course be used instead.Such a system, however, defies the injection ratio control for eachcylinder. The same effect can still be implemented using a combustionstate control of other form.

It has been mentioned that the CVT (continuously variable transmission),with which a required speed can easily be achieved, should preferably beused for the transmission 17. However, the combustion state controldescribed for the embodiment of the present invention can still berealized even with a stepped gear transmission (or manual transmissionor MT) and a stepped gear automatic transmission (or automatictransmission or AT).

FIG. 8 is a system configuration diagram showing an engine controlapparatus according to a second embodiment of the present invention. Acylinder pressure sensor 23 is used as the sensor for providing an inputsignal for the combustion state estimating means for each cylinder.Providing the cylinder pressure sensor 23 for each cylinder naturallydrives up cost. It, however, allows the control apparatus to estimatethe combustion state in each cylinder even more accurately. An engineoperation at even higher efficiency with even lower exhaust emissionscan therefore be achieved as compared with the control apparatusaccording to the first embodiment of the present invention.

FIG. 9 is a system configuration diagram showing an engine controlapparatus according to a third embodiment of the present invention. Arotary electric motor (hereinafter referred to as the “motor”) 24connected by way of gears 25, 27 to the output shaft 15 of the engine isused as the sensor for providing an input signal for the combustionstate estimating means for each cylinder. The motor 24 allows the enginetorque during the compression ignition combustion mode to be detectedhighly accurately, allowing the system to estimate the combustion stateof each cylinder.

In addition, the motor is ordinarily capable of assisting in the enginetorque as employed in a hybrid vehicle or suppressing variations intorque during gear-shifting or combustion mode switching. It thus offersa great benefit of introduction as a system.

FIG. 10 is a system configuration diagram showing an engine controlapparatus, according to a fourth embodiment of the present invention,mounted on a generator. The generator shown in FIG. 10 is provided witha motor generator 21 and an inverter 22. The engine 12 may be configuredas that described in the first, the second, or the third embodiment.

Referring to FIG. 10, the output from the engine 12 is supplied by wayof the motor generator 21 and the inverter 22 to an external apparatusas electric power. The PCU 1 reads external information D (such asweather conditions, circuit conditions, power requirements and the like)while the equipment is running. The signal fed from a speed ratiosetting means 4 a to the inverter 22 is used, at this time, to controlthe input speed and the output frequency, thereby controlling the enginespeed. This allows the combustion state of each cylinder to be estimatedand controlled as described earlier. That is, the application of thecontrol apparatus according to the fourth embodiment of the presentinvention to the generator makes possible equipment operations at highefficiency with low exhaust emissions, just as in the embodimentsdescribed heretofore.

Although the present invention has been described in connection withwhat are conceived to be practical and preferred embodiments, it is tobe understood that departures may be made therefrom within the spiritand scope of the invention, which is not limited to the illustrativedetails disclosed.

As can be understood from the foregoing descriptions, the controlapparatus for a multi-cylinder engine according to the preferredembodiments of the present invention is capable of appropriatelycontrolling the cylinder pressure peak value and peak timing of eachcylinder during the engine operation in the compression ignitioncombustion mode. This can suppress variations in the combustion stateamong different cylinders and different cycles during the engineoperation in the compression ignition combustion mode, thus realizingcompression ignition combustion offering good fuel consumption andexhaust emissions characteristics.

1. A control apparatus for a multi-cylinder engine capable of performingcompression ignition combustion, or selecting either the compressionignition combustion or spark ignition combustion, comprising: acylinder-specific combustion state estimating means for estimating acombustion state for each cylinder; and a combustion state control meansfor controlling the combustion state for each cylinder; wherein thecombustion state control means controls, during an engine operation in acompression ignition combustion mode, the combustion state for eachcylinder based on the combustion state estimated by thecylinder-specific combustion state estimating means during the engineoperation in the compression ignition combustion mode, wherein thecombustion state control means controls at least one of a variable valvemechanism that varies an amount of internal EGR by changing at least oneof the valve timing and an amount of valve lift of an intake valve andan exhaust valve, a fuel injection mechanism that directly or indirectlyinjects fuel into a combustion chamber of the multi-cylinder engine, anda transmission that transmits output shaft torque of the multi-cylinderengine to a power train mechanism of a vehicle through adjusting a gearratio, and the combustion state control means controls the transmissionso as to change the gear ratio, thereby decreasing an engine speed whenit is estimated that the peak value of cylinder pressure of at least oneof the plurality of cylinders is lower than a predetermined value, orthe peak timing of cylinder pressure is retarded in relation topredetermined timing, based on the estimation made by thecylinder-specific combustion state estimating means.
 2. The controlapparatus for the multi-cylinder capable of performing compressionignition combustion, or selecting either the compression ignitioncombustion or spark ignition combustion, comprising: a cylinder-specificcombustion state estimating means for estimating a combustion state foreach cylinder; and a combustion state control means for controlling thecombustion state for each cylinder; wherein the combustion state controlmeans controls, during an engine operation in a compression ignitioncombustion mode, the combustion state for each cylinder based on thecombustion state estimated by the cylinder-specific combustion stateestimating means during the engine operation in the compression ignitioncombustion mode, wherein the combustion state control means controls atleast one of a variable valve mechanism that varies an amount ofinternal EGR by changing at least one of the valve timing and an amountof valve lift of an intake valve and an exhaust valve, a fuel injectionmechanism that directly or indirectly injects fuel into a combustionchamber of the multi-cylinder engine, and a transmission that transmitsoutput shaft torque of the multi-cylinder engine to a power trainmechanism of a vehicle through adjusting a gear ratio, and thecombustion state control means controls the transmission so as to changethe gear ratio, thereby increasing the engine speed when it is estimatedthat the peak value of cylinder pressure of at least one of theplurality of cylinders is higher than the predetermined value, or thepeak timing of cylinder pressure is advanced in relation to thepredetermined timing, based on the estimation made by thecylinder-specific combustion state estimating means.
 3. The controlapparatus for the multi-cylinder engine according to claim 2, whereinthe combustion state control means increases an amount of fuel suppliedfrom the fuel injection mechanism to each cylinder per one cycle.
 4. Thecontrol apparatus for the multi-cylinder engine according to claim 1,wherein the combustion state control means decreases an amount of fuelsupplied from the fuel injection mechanism to each cylinder per onecycle.
 5. A control apparatus for the multi-cylinder engine capable ofperforming compression ignition combustion, or selecting either thecompression ignition combustion or spark ignition combustion,comprising: a cylinder-specific combustion state estimating means forestimating a combustion state for each cylinder; and a combustion statecontrol means for controlling the combustion state for each cylinder;wherein the combustion state control means controls, during an engineoperation in a compression ignition combustion mode, the combustionstate for each cylinder based on the combustion state estimated by thecylinder-specific combustion state estimating means during the engineoperation in the compression ignition combustion mode, wherein, if thecylinder-specific combustion state estimating means estimates that acylinder pressure peak value of at least one cylinder out of theplurality of cylinders falls outside a predetermined range, or thecylinder pressure peak timing of at least one of the plurality ofcylinders falls outside a predetermined period of time, the controlapparatus inhibits the compression ignition combustion mode for themulti-cylinder engine, or switches the operating mode to the sparkignition combustion mode, and warning equipment is operated if thecompression ignition combustion mode is inhibited.
 6. A control methodfor the multi-cylinder engine that performs compression ignitioncombustion control or provides control so as to select either thecompression ignition combustion or spark ignition combustion, comprisingthe steps of: estimating, during an engine operation in the compressionignition combustion mode, a combustion state for each cylinder duringthe engine operation in the compression ignition combustion mode; andcontrolling the combustion state of each cylinder based on the estimatedcombustion state for each cylinder, wherein the step of estimating thecombustion state of each cylinder is to estimate a peak value ofcylinder pressure of each cylinder and peak timing of the cylinderpressure and the step of controlling the combustion state is to ensurethat the peak value of the cylinder pressure of each cylinder fallswithin a predetermined range and/or the peak timing falls within apredetermined period of time after piston top dead center, and thecylinder pressure of each cylinder represents a value that is the resultof analyzing a vibration frequency detected of the multi-cylinderengine.
 7. A control method for a multi-cylinder engine that performscompression ignition combustion control or provides control so as toselect either compression ignition combustion or spark ignitioncombustion, comprising estimating, during an engine operation in thecompression ignition combustion mode, the combustion state for eachcylinder during the engine operation in the compression ignitioncombustion mode; and controlling the combustion state by allowing avariable valve mechanism of the cylinder to increase or decrease a minusoverlap amount so as to increase or decrease an amount of internal EGR,if at least either one of the following conditions is estimated, theconditions being that a peak value of the cylinder pressure of at leastone cylinder of all the plurality of cylinders is lower or higher than apredetermined value, and that a peak timing of the cylinder pressure isretarded or advanced in relation to predetermined timing, and allowing atransmission to change a gear ratio so as to increase or decrease anengine speed, thereby increasing or decreasing the amount of fuelsupplied from a fuel injection mechanism to each cylinder per one cycle,if at least either one of the following conditions is estimated, theconditions being that the peak value of the cylinder pressure of atleast one cylinder of all the plurality of cylinders is lower or higherthan a predetermined value, and that the peak timing of the cylinderpressure is retarded or advanced in relation to a predetermined timing.8. The control method for the multi-cylinder engine according to claim6, wherein other combustion states including a mode changeover betweenspark ignition combustion and compression ignition combustion iscontrolled based on the operating condition of the multi-cylinderengine, the operating condition of a piece of equipment mounted with themulti-cylinder engine and, an intention of a user of the equipment.